One of the principal problems found with medical X-ray targets is one of warpage of the focal track. While some slight warpage, whether of the concave or convex type, can be tolerated, less than can be detected with the naked eye causes an undesirable drop-off in X-ray output. With warpage of the focal track, X-rays are cut off at the periphery of the X-ray window in the surrounding tube enclosure. As detected by the center position of the X-rays on the external film, if this spot shifts as little as 1.degree., an X-ray deficiency can exist. With certain target designs and exposures, this can occur in less than 1,000 exposures whereas the X-ray tubes are typically guaranteed for 10,000 exposures. This warpage becomes more severe or occurs earlier as the target diameter increases and the overall temperature of the substrate rises. It is this problem to which this invention is directed and it has been found that this warpage can be minimized by providing certain alloys of molybdenum as the substrate.
British Pat. No. 1,121,407 issued July 24, 1968 and assigned to Metallwerk Plansee Aktiengesellschaft discloses the use of an X-ray base comprising molybdenum alloyed with titanium and/or zirconium and optionally carbon. Other patents such as U.S. Pat. No. 3,649,355 Hennig issued March, 1972 and assigned to Schwarzkopf Development Corporation disclose use of a graphite base, U.S. Pat. No. 3,660,053 Palme issued May 2, 1972 discloses use of a molybdenum base and a commercial X-ray tube is marketed with a base of molybdenum alloyed with 5% tungsten.